Cylinder actuators are used to actuate loads. The cylinder may impart a force while either extending or retracting which in turn moves the load respectively. This operation can be performed with several different styles of actuators including electro-mechanical, pneumatic, and hydraulic. Various valves control the fluid circuits in hydraulic systems.
An electro-hydraulic actuator (EHA), for example, functions in a bi-rotational design; the direction of the electric motor rotation and thus hydraulic pump flow is determined by the electric polarity applied to the motor leads. Thus when the polarity is applied in the extend direction, the pump will produce flow to the piston side of the cylinder. As the cylinder extends, the fluid from the rod side of the cylinder will return to the pump and flow to the piston side of the cylinder.
Conversely, when electrical polarity is applied in the reverse direction, the pump will reverse rotation and pump flow to the rod side of the cylinder. As such, the piston side fluid returns to the pump to be delivered to the rod side of the cylinder.
As the EHA is designed to exert force in both extend and retract directions, there is consideration made for over pressure valves with, for example, one valve dedicated to each side of the hydraulic circuit (extend and retract respectively). These relief valves are designed to limit the operating pressure to a safe level preventing mechanical damage to the EHA components during operation.
Additional valving is also common to a double-acting hydraulic circuit to compensate for the issue of differential areas between the rod and piston sides of the cylinder. Because the cylinder rod may be present on one side of the cylinder and not the other, there is a difference in hydraulic volume between the piston side and rod side. Therefore as the cylinder extends, the volume of fluid returning from the rod side is insufficient and additional oil must be drawn from tank via a check valve. As the cylinder retracts, excessive fluid returns to the pump and thus needs to be dumped to a reservoir. This can be accomplished via several designs.
Conventional designs will use the retract relief valve to dump this fluid to tank.
Another design utilizes an additional relief valve set to a minimal pressure in combination with a pilot operated check valve to allow the excess fluid to return to tank.